1. Field of the Invention
This invention deals with photography and, more specifically, with novel products and processes wherein the spectral response of various photosensitive materials is extended by the use of novel linked spectral sensitizing dyes.
2. Description of the Prior Art
As understood in the art and as herein used, the term "spectral sensitizing dyes" refers to compositions which extend the response of photosensitive materials to radiation to which the material normally is substantially insensitive. In general, the procedure of spectral sensitization of photosensitive material, and preferably sensitization of photosensitive silver halide, is accomplished by the adsorption onto one or more surfaces of the photosensitive material of one or more of the aforesaid spectral sensitizing dyes selected from certain classes of dyes including preferably, cyanine dyes and dyes related to them. For an extensive treatment of cyanine and related dyes particularly adapted to provide spectral sensitization of, for example, a photosensitive silver halide crystal, see Hamer, F. M., The Cyanine Dyes And Related Compounds, Interscience Publishers, New York, New York, U.S.A. (1964); or, Mees, C. E. K., and James, T. H., The Theory Of The Photographic Process, 3rd Edition, the Macmillan Co., New York, New York, U.S.A. (1966), pp. 198-232.
Cyanine dyes and related compounds are well known particularly in the art of photographic chemistry. The most common "true" cyanine dyes have a chromophoric structure which conforms to an amidinium ion system in which two nitrogen atoms are included in separate heterocyclic ring systems connected by a conjugated chain of methine groups. The aforementioned dyes are usually prepared by condensation reactions, conducted in the presence of a basic condensing agent, involving a pair of heterocyclic quaternary salts, one having at least one active methyl substituent and the other having at least one proton-seeking (negative) group capable of condensing with the aforementioned methyl substituent.
In compounds related to the cyanines, the chromophoric structure of the dye is similar to that of the true cyanines just described, but may include a carboxyl ion system or amidic system wherein either or both of the two nitrogen atoms of the amidinium ion system is replaced with an oxygen atom. The merocyanine dyes are an example of cyanine-related compounds wherein a typical cyanine dye heterocyclic nucleus is linked through a conjugated methine bridge with a nucleus derived from an acidic ketomethylene compound, e.g., 3-ethylrhodanine.
Various "linked" (i.e., "bis", "polymeric", "pseudopolymeric", "double", "poly-nuclear", etc.) cyanine dyes have been described in the photographic and chemical arts as spectral sensitizers. In the patent literature, such linked cyanine compounds have been described in, for example, U.S. Pat. Nos. 2,393,351; 2,425,772; 2,425,773; 2,425,774; 2,518,731; 2,641,137; 2,592,196; and 3,622,317. In the prior art represented by the above-cited patents, a hydrocarbon chain, e.g., an alkylene chain joining one amidinium nitrogen of one cyanine dye segment to that of another, a poly-methine chain joining carbon atoms, etc., is commonly used as the linking group. Such prior art linked cyanine dye compounds often involve complicated or impractical syntheses which, in many instances, result in a mixture of products from which it is difficult to separate a pure product.
Novel linked sensitizing dye compounds are herein provided which do not exhibit the above-described deficiencies of the prior art and can be readily prepared in excellent yields.